Is there a method to decrease a DSO sampling rate intentionally so that longer capture window times can be achieved [at one-shot capture mode] ?

For example, given a 1Mb/s max digital data input, and a 1G Sa/s scope with a 100Mpts memory depth. A one shot capture at full memory depth will span a 100ms time window. Is there a way to intentionally decrease the sampling rate to, say, 10M Sa/s (which is enough for sampling a 1 Mb/s data), and with the same memory depth get a span x100 (1G Sa/s / 10M Sa/s) folds more - e.g. 10 Seconds ?

Is there a way to intentionally decrease the sampling rate to, say, 10M Sa/s (which is enough for sampling a 1 Mb/s data), and with the same memory depth get a span x100 (1G Sa/s / 10M Sa/s) folds more - e.g. 10 Seconds ?

The scopes adjust the sampling rate to suit the timebase speed and the capture memory size.

For a particular scope, it depends on the scope firmware. Here is the way the firmware of a Rigol DS1052E is set for sample rate versus timebase settings + memory depth).